I'm not sure, I haven't heard back from a few schools to be honest but I've been reading that a lot of schools don't inform their candidates that they're on a waitlist, so they wait to hear from the first round of accepted students to see if they accept their offers before contacting the "un-contacted" candidates. I assume that they wait then inform those candidates after if there's space so as to not give them any unnecessary hope just in case it doesn't work out, but I'm not quite sure. But the unofficial waitlist thing seems to be a standard practice among PHD programs I've heard! I guess it's better than having outright received a rejection letter crossing my fingers for you and everyone else!